Sankaran Mahesh, Hanan Niall P, Scholes Robert J, Ratnam Jayashree, Augustine David J, Cade Brian S, Gignoux Jacques, Higgins Steven I, Le Roux Xavier, Ludwig Fulco, Ardo Jonas, Banyikwa Feetham, Bronn Andries, Bucini Gabriela, Caylor Kelly K, Coughenour Michael B, Diouf Alioune, Ekaya Wellington, Feral Christie J, February Edmund C, Frost Peter G H, Hiernaux Pierre, Hrabar Halszka, Metzger Kristine L, Prins Herbert H T, Ringrose Susan, Sea William, Tews Jörg, Worden Jeff, Zambatis Nick
Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, Colorado 80523, USA.
Nature. 2005 Dec 8;438(7069):846-9. doi: 10.1038/nature04070.
Savannas are globally important ecosystems of great significance to human economies. In these biomes, which are characterized by the co-dominance of trees and grasses, woody cover is a chief determinant of ecosystem properties. The availability of resources (water, nutrients) and disturbance regimes (fire, herbivory) are thought to be important in regulating woody cover, but perceptions differ on which of these are the primary drivers of savanna structure. Here we show, using data from 854 sites across Africa, that maximum woody cover in savannas receiving a mean annual precipitation (MAP) of less than approximately 650 mm is constrained by, and increases linearly with, MAP. These arid and semi-arid savannas may be considered 'stable' systems in which water constrains woody cover and permits grasses to coexist, while fire, herbivory and soil properties interact to reduce woody cover below the MAP-controlled upper bound. Above a MAP of approximately 650 mm, savannas are 'unstable' systems in which MAP is sufficient for woody canopy closure, and disturbances (fire, herbivory) are required for the coexistence of trees and grass. These results provide insights into the nature of African savannas and suggest that future changes in precipitation may considerably affect their distribution and dynamics.
稀树草原是对人类经济具有重大意义的全球重要生态系统。在这些以树木和草本植物共同占主导地位为特征的生物群落中，木本植物覆盖度是生态系统属性的主要决定因素。资源（水、养分）的可利用性和干扰机制（火灾、食草作用）被认为在调节木本植物覆盖度方面很重要，但对于这些因素中哪一个是稀树草原结构的主要驱动因素，人们的看法存在差异。在这里，我们利用来自非洲854个地点的数据表明，年平均降水量（MAP）小于约650毫米的稀树草原中，最大木本植物覆盖度受MAP限制，并随MAP呈线性增加。这些干旱和半干旱稀树草原可被视为“稳定”系统，其中水分限制木本植物覆盖度并允许草本植物共存，而火灾、食草作用和土壤性质相互作用，使木本植物覆盖度降低至MAP控制的上限以下。在MAP约650毫米以上，稀树草原是“不稳定”系统，其中MAP足以使木本植物冠层闭合，树木和草本植物共存需要干扰（火灾、食草作用）。这些结果为非洲稀树草原的性质提供了见解，并表明未来降水量的变化可能会极大地影响其分布和动态。
